Electron optics of traditional CRTs are constructed of mechanical electrodes having apertures to which voltages are applied. It is also known to supply currents to coils to create magnetic fiels for shaping the electron beam of a CRT. These systems alter the electron beam trajectory with electrostatic or magnetic fields.
In prior art CRTs, the electron beam is generated from a thermoinic dispenser or field-emitting cathode. The beam is accelerated and focused to a first crossover (object point) by first and second accelerator electrodes. The apertures in these electrodes are known to have either a circular cross-section or a two-axis (rectangular) cross-section and provide a desired shape, or projected energy profile, of the electron beam in the plane perpendicular to its longitudinal axis. The projected energy profile at an object point correspondingly will have a circular or elliptical profile in a plane perpendicular to the longitudinal axis of the electron beam. In both cases, the electron beam has a closely Gaussian energy distribution along each axis perpendicular to the longitudinal axis of the electron beam.
The beam is then focused to an image point on the phosphor screen using traditional main focusing lenses, which are either electrostatic or magnetic. The apertures of these lenses have a circular cross-section and provide a magnified or minified image point energy profile of the object point energy profile. Elliptical object point projected energy profiles are used to match the color stripes of color CRTs. Circular object point projected energy profiles are used for high-resolution monochrome CRTs.
It is also known to dynamically correct for astigmatism of projected energy profiles in a plane that is not perpendicular to the longitudinal axis of the electron beam by altering the projected energy profile of an electron beam in a plane perpendicular to its longitudinal axis by using a focusing lens placed along the path of the electron beam. Astigmatism occurs when the angle of the deflection of an electron beam is high enough to strike the outer edges of a large phosphor screen. Although an electron beam generally maintains the shape of its projected energy profile in a plane perpendicular to its longitudinal axis, the edges of a large phosphor screen are far from being perpendicular to the longitudinal axis of the electron beam. Thus, the energy profile projected onto the edge of a phosphor screen is different than the energy profile projected in a plane perpendicular to its longitudinal axis. As the deflection angle of the electron beam increases, so does the amount of distortion, or astigmatism, to the projected energy profile of the electron beam. Others have suggested correcting for astigmatism by using different focusing lenses comprising a series of electrodes to alter the profile of the electron beam in the plane perpendicular to its longitudinal axis before deflection. (See U.S. Pat. Nos. 5,164,640 to Son et al.; 5,055,749 to Chen et al.; 5,036,258 to Chen et al.; 4,978,886 to Miyamoto et al; and 4,814,670 to Suzuki et al.) Thus, the desired image point projected energy profile is projected on the edges of the phosphor screen.
It is also known to use a single CRT to display images from different imaging sources, such as charge-coupled device cameras (CCD cameras) and infrared cameras (IR cameras). Unfortunately, CCD and IR cameras normally have different resolutions and/or aspect ratios. CCD cameras traditionally have high horizontal and vertical resolution (e.g., 640 pixels.times.480 pixels) whereas IR cameras traditionally have high horizontal resolution but low vertical resolution (e.g., 640 pixels.times.120 pixels). An IR image may only use one-quarter of the vertical resolution of a CRT designed for a CCD camera, thus leaving dark horizontal bands on the CRT. This "venetian blind" effect is distracting and reduces the usefulness of the displayed IR image. Accordingly, it would be useful to provide a CRT capable of displaying images from sources having dissimilar resolutions and/or aspect ratios.
Heads-up-display (HUD) systems are well known. They comprise a CRT which projects an image onto a transparent carrier through which a pilot/driver views a scene. The projected image may include data such as navigational information, weapon status information, targeting information and terrain enhancement. The pilot/driver sees the projected image as part of the scene viewed. However, the different types of data typically provided by HUD systems are best delivered at different resolutions and brightness. For example, alphanumeric information is presented in high brightness and medium resolution, (e.g., 320 pixels.times.240 pixels) and is drawn using a stroke (vector) mode display. This allows the alphanumeric information to be easily viewed even when the pilot/driver is surrounded by bright light. Terrain enhancement is accomplished by displaying an IR camera image at lower brightness, high-horizontal resolution and low-vertical resolution (e.g., 640 pixels.times.120 pixels), and by using a raster scan (line scan) mode display. Finally, CCD images are best presented in low brightness and high resolution (e.g., 640 pixels.times.480 pixels), and by using a raster scan (line scan) mode display. The prior art teaches using a different CRT to provide each type of image to a HUD system.
In addition, it is known to display more than one type of information on a HUD system simultaneously using sources having different display characteristics. For example, it may be necessary for the pilot/driver to use both IR terrain enhancement and alphanumeric navigation and targeting information concurrently. The multiple CRT solution of the prior art is capable of providing multiple displays simultaneously to a HUD system where each CRT provides a display of varying resolution, brightness and mode. Unfortunately, multiple CRT systems are expensive and susceptible to mechanical and electrical failure. It would be advantageous to provide a single CRT that could switch between multiple display modes having different characteristics to display images in rapid succession such that the human eye would perceive the projected images as being presented simultaneously.